I'm stuck on this question:

A light inextensible string AB of length 3l has its ends fixed to two points A and B which are in a vertical line with A a distance l above B. A smooth ring of mass m is threaded on the string and is made to move in a horizontal circle centre B with a constant speed. Calculate the tension in he string and the speed of the ring.

When I tried to calculate th tension I found that I needed either the radius of the circle of the diatance from either A or B to the ring, which I don't have. Have I missed something in the question? Could someone help please?